The present invention relates to a gas analyzer, for example an infrared gas analyzer, of fluid modulation type, in which a test or sample gas and a standard gas are alternately introduced at predetermined periods of time or intervals or cycles into at least one cell. For example, the test gas and standard gas may alternately be introduced into a single cell or into a pair of cells. That is to say, when the test gas is introduced into one cell, the standard gas is introduced into the other cell.
In a gas analyzer of the fluid modulation type, changes in the variation of the energy detected by a detector are influenced by a change in the time required for a displacement of the test gas contained in a cell by the standard gas, and vice versa. For example, the displacement of these gases in a cell occurs in a pulse-like manner. The detector thus operates at highest modulation efficiency, which is theoretically ideal, when the flow rate K of the test gas and the standard gas is infinitely great. On the contrary, the detector operates at a modulation efficiency of zero when the flow rate K of the test gas and the standard gas is equal to one half of the volume of a cell per gas displacement period (see FIG. 1).
Now assuming that diffusion does not take place at the boundary between the standard gas and the test gas and that displacement is carried out immediately before a cell, the modulation efficiency S can be expressed by the following equation if it is assumed that the modulation efficiency is an integral value of the displacement or change of the gas in the cell (see FIG. 2): ##EQU1## wherein: K=QT/2V;
K.gtoreq.0.5; PA1 Q=Flow rate of the test gas and the standard gas per unit time; PA1 V=Volume of the cell; and PA1 T=Modulation period (two times the gas-displacement period).
FIG. 3 shows dimensionless output-flow rate characteristics for cells having various shapes. The shapes of cells graphed or plotted in FIG. 3 are as follows:
- - - .phi.8.times.100; - - - .phi.10.times.100; - - - .phi.12.times.100; - - - .phi.12.times.50 (mm). It will be clearly understood that the ideal curve from the above described equation (1), as shown in FIG. 2 and as shown by the curve in FIG. 3, is sufficiently identical with practical output-flow rate characteristics, as shown by the plottings in FIG. 3. This shows that the gas displacement efficiency has an influence upon the output from the detector. Also, a differential value of this curve gives the value of the influence upon the output of the detector by variations or changes of the flow rate of the test gas and the standard gas.
Moreover, FIG. 4 and FIG. 5 are graphs showing the test results of changes in flow rate for a double cell of 3 mm and 35 mm, respectively. Curves a, a' show the detector outputs for various values of the flow rate of span gas, i.e. a gas having a known concentration used for calibration in place of a sample gas, using nitrogen as the standard gas at a constant flow rate of 0.5 liter/min. Hereinafter b, b'; c, c'; d, d'; e, e'; and f, f' show the detector outputs employing nitrogen as the standard gas at constant flow rates of 1.0 liter/min, 2.0 liter/min, 3.0 liter/min, 4.0 liter/min and 5.0 liter/min, respectively. It clearly will be understood that the variations in the flow rate of the gas greatly influences the output of the analyzer.
As clearly understood from the above description, in a gas analyzer of the fluid modulation type, the maintenance of a constant flow rate of gas introduced into the cell is most important for eliminating errors in the output of the analyzer.
In a conventional analyzer of this type, a constant pressure regulator and a capillary tube are used in order to attempt to introduce the gas into the cell at a constant flow rate. It has, however, been found that the accurate maintenance of a constant flow rate over a long period of time is impossible for the following reasons:
a. poor mechanical reproducibility of a constant pressure regulator;
b. time change of pressure adjustment due to contamination of the constant pressure regulator; and P. c. choking and contamination of a capillary tube or the like, with the result that indication errors are unavoidable.